Professional Pilot, July 2019

Forecast model estimates of the likelihood of strong to severe clear air turbulence CAT over the North Pacific Ocean CAT frequently occurs in and around the jet stream at the top of the troposphere which can be a challenge when pilots seek the strong tailwinds the jet stream also offers PROFESSIONAL PILOT July 2019 63 kts 30 m s Those regions are jet streams What this all means is that flight at higher flight levels is still prone to intercepting a great deal of weather including turbulence icing and even hail Turbulence When we think of high altitude cruising the 1st thing that most pilots consider is the jet stream and whether they will take advantage of screaming tail winds on an eastbound flight or avoid its fuel gulping headwinds on westbound journeys However the presence of fast moving air embedded in a region of more moderate winds often leads to strong to extreme turbulence As the jet stream shifts direction or speed even slightly it shears against the slower moving air surrounding it These turbulent eddies can shear in any direction and may extend several kilometers away from the jet axis In general jet stream turbulence also known as clear air turbulence CAT is most likely to occur and is strongest on the cold side of the jet axis with secondary areas of concern above and below the axis CAT is experienced less frequently on the warm side of the jet axis Although all CAT associated with a jet stream should be addressed with caution its intensity and extent are normally proportional to the speed of winds in the jet axis the width of the jet and the degree of curvature in the flow The area around jet streaks where winds may exceed 150 kts and which often occur within the base of a sharply curving trough is a likely location for encounters with extreme CAT Jet streams also often merge or separate and these actions frequently produce a region of CAT Another CAT factor to consider is that modern aviation technology has placed very large aircraft on common routes in the high flight levels These aircraft produce significant wake turbulence while they cruise Because these flight routings often have minimal crosswind the turbulent vortices from these heavy jets descend into the paths of smaller aircraft on the same routes at lower flight levels Where possible pilots should request ATC alerts if a heavy aircraft will overfly them Finding favorable winds Finding favorable or avoiding unfavorable winds in the jet stream is another high altitude weather consideration Often this means finding the location of a jet stream that may only be a few hundred km wide and less than 10000 ft thick Upper air jets in the middle latitudes migrate through the year both horizontally and vertically In the winter the polar air mass expands pushing the polar front equatorward to around 35 40 degrees latitude The colder temperature of both the polar and subtropical air masses also means the winter polar jet occurs at between FL250 and FL300 In the summer the polar jet shifts to around 50 60 degrees latitude and rises to between FL300 and FL390 The weaker subtropical jet can be as high as FL500 in the summer Synoptic pressure variations also help to propagate short and long wave patterns within the jet These patterns can have extreme amplitudes and wavelengths meaning that the ordinary westerly airflow may appear nearly northerly or southerly as it passes through these waves Thus pilots could find themselves crossing through a jet even though the general position of the polar front might be hundreds of km away In general winds will gradually increase in speed with height in the troposphere with the strongest winds and the jet stream in the tropopause As soon as one climbs into the stratosphere wind speeds quickly diminish To optimize fuel burn and travel time westbound pilots may need to choose a cruise altitude that keeps them out of the tropopause while eastbound pilots will seek exactly the opposite Icing Although convection is largely limited to the troposphere we must remember that especially in the tropics or during the summertime the top of the troposphere still exceeds the ceilings of most aircraft In the intertropical convergence zone towering cumuli routinely rise to between 45000 and 60000 ft Not only are these towering cells nearly impossible to overfly the flight levels that will take aircraft through their upper reaches are well above the 15000 ft 4500 m freezing level height Furthermore these clouds

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